Abstract
The veb1 gene cassette encodes the extended spectrum β-lactamase, VEB-1 that is increasingly isolated from worldwide Gram-negative rods. Veb1 is commonly inserted into the variable region of different class 1 integrons in which it is always associated with a downstream-located aadB gene cassette encoding an aminoglycoside adenylyltransferase. In Pseudomonas aeruginosa, the majority of veb1-containing integrons also carry an insertion sequence, IS1999 that is inserted upstream of the veb1 gene cassette and disrupts the integron specific recombination site, attI1. Investigation of the recombination properties of the sites surrounding veb1 revealed that insertion of IS1999 reduces significantly the recombination frequency of attI1 and that veb1 attC is not efficient for recombination in contrast to aadB attC. Subsequent sequence optimisation of veb1 attC by mutagenesis, into a more consensual attC site resembling aadB attC, successfully improved recombination efficiency. Overall, this work gives some insights into the organisation of veb1-containing integrons. We propose that IS1999 and the nature of veb1 attC stabilize the veb1 gene cassette environment likely by impairing recombination events upstream or downstream of veb1, respectively.
Highlights
Class 1 integrons are increasingly reported as a reservoir for antibiotic resistance genes in Gram-negative rods [1,2]
The 59 conserved segment (59-CS) classically includes a gene encoding a site-specific recombinase of the DNA integrase family, intI1, the cassette integration site, attI1, and the promoter Pc, which is oriented toward the integration point of the gene cassettes and is responsible for gene cassette expression [2,3,4,5,6]
Gene cassettes from the variable region are composed of a gene, usually an antibiotic resistance gene, and a downstream recombination site known as attC site or 59-base element (59-be) [1,2,3]
Summary
Class 1 integrons are increasingly reported as a reservoir for antibiotic resistance genes in Gram-negative rods [1,2]. These structures possess two conserved regions located on each side of a variable region consisting of integrated gene cassettes [1,2,3] (Figure 1A). The 59 conserved segment (59-CS) classically includes a gene encoding a site-specific recombinase of the DNA integrase family, intI1, the cassette integration site, attI1, and the promoter Pc, which is oriented toward the integration point of the gene cassettes and is responsible for gene cassette expression [2,3,4,5,6]. The further the cassette is relative to Pc, the lower its expression will be [4]
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